1. Field of the Invention
The present invention relates to a communications navigation system in which navigation data necessary for route guidance is supplied from a navigation base to a moving body such as a vehicle, and also relates to a navigation base apparatus provided at the navigation base and a navigation apparatus to be mounted on the vehicle (hereinafter, referred to as "vehicle navigation apparatus" or "navigation apparatus") for use in the communications navigation system.
2. Description of the Prior Art
In the navigation systems commonly used today, a vehicle such as an automobile is provided with a navigation apparatus, for instance. In such navigation systems, route guidance is carried out by utilizing map data stored in a CD-ROM or the like provided in connection with the navigation apparatus. However, such systems require the user thereof to constantly buy new CD-ROMs in order to obtain new information on changing conditions such as the construction of new roads and the abolition of old roads. Further, when the type of recording medium is changed (e.g., to DVD-ROM) or when a different format is used with the same type of recording medium, the navigation apparatus must be replaced with other one that is suited for such new type of recording medium or format.
In view of such problems, Japanese Laid-Open Patent Application No. HEI 10-19588 discloses a communications navigation system in which map images and optimum route data necessary to guide a vehicle to a destination are transmitted from a navigation base (center) to a vehicle such as an automobile. Namely, in this system, communication is carried out between a data transmission system in the navigation base and a vehicle navigation apparatus mounted on the vehicle. In this regard, the data transmission system includes a data base which stores data necessary for guiding a vehicle to a destination, and based on a request from the vehicle navigation apparatus, the data transmission system reads out necessary data from the data base to produce map images. Further, the data transmission system carries out a route search to produce optimum route data. After these operations are completed, the data transmission system transmits data such as the produce map image data and optimum route data to the vehicle navigation apparatus. Then, the vehicle navigation apparatus displays guidance information based on the map image data and optimum route data transmitted from the data transmission system.
In the above-described communications navigation system, all of route data and map image data from the departure point to the destination produced at the navigation base are transmitted to the vehicle navigation apparatus of the vehicle as they are. In the vehicle, map images corresponding to the present position of the vehicle are successively displayed on the display of the vehicle navigation apparatus allover the route including the departure point and the destination.
However, the above-described communications navigation system involves the following problems.
First, as described above, in the above-described communications navigation system, all of the route data and map images from the departure point to the destination are transmitted to the vehicle. Therefore, the amount of the data to be transmitted becomes huge if the distance between the departure point and the destination is long, so that it takes a long time to transmit the data. Further, it is necessary for the navigation apparatus to temporally store the huge amount of the transmitted data. However, actually, the driver of the vehicle does not always view the display of the navigation apparatus while driving. In most cases, the driver needs to have assistance of the navigation apparatus at the main points of the route such as intersections to be turned to right or left and the like. Thus, with regard to intersections through which the driver is to proceed straight, it is not necessary for the driver to make reference to the vehicle navigation apparatus. This means that it is possible to carry out navigation if there are route data (guidance information) and map images only for the main points of the route at which guidance is needed, such as the intersections and forks where the driver is to turn right or left. Further, if the navigation base is adapted to transmit only such data necessary for guidance to the vehicle, it becomes possible to shorten a time necessary for the data transmission and considerably reduce the amount of data to be stored in the vehicle navigation apparatus, and this in turn makes it possible to simplify the structure of the vehicle navigation apparatus.
Further, there is a case that the driver knows the recommended route very well. In such a case, it is sufficient for the driver if a simple map image of the route is obtained, and such a simple route guidance may be more preferable to the driver.
Moreover, in the above-described communications navigation system, if the vehicle strays from the recommended route to the destination, it will not be possible to continue route guidance. In such a case, the driver of the vehicle has to transmit the current position and destination data to the navigation base again. Then, the navigation base carries out route search to the destination again based on the current position and destination data and establishes another recommended route, and then transmits its data to the vehicle so that guidance is carried out again after such recommended route is received by the vehicle navigation apparatus. Consequently, because route guidance to the destination is suspended for a short period of time, driver distress can occur.
On the other hand, even in the case where the vehicle navigation apparatus is provided with the map data stored in the CD-ROM or the like, it becomes no longer possible to continue route guidance once the vehicle strays from the established recommended route. In such a case, similar to the above situation, it is necessary to carry out a route search again using the CD-ROM to locate another recommended route to the destination. However, this also requires suspension of route guidance during the time another route is being searched, so that driver distress may occur.